diff --git a/lib/Models/SendingQueue.php b/lib/Models/SendingQueue.php
index 331fe15ea1..bf8512c599 100644
--- a/lib/Models/SendingQueue.php
+++ b/lib/Models/SendingQueue.php
@@ -65,6 +65,20 @@ class SendingQueue extends Model {
     return $this;
   }
 
+  /**
+   * Used only for checking processed subscribers in old queues
+   */
+  private function getSubscribers() {
+    if(!is_serialized($this->subscribers)) {
+      return $this->subscribers;
+    }
+    $subscribers = unserialize($this->subscribers);
+    if(empty($subscribers['processed'])) {
+      $subscribers['processed'] = array();
+    }
+    return $subscribers;
+  }
+
   function getNewsletterRenderedBody($type = false) {
     $rendered_newsletter = $this->decodeRenderedNewsletterBodyObject($this->newsletter_rendered_body);
     return ($type && !empty($rendered_newsletter[$type])) ?
@@ -95,8 +109,18 @@ class SendingQueue extends Model {
   }
 
   function isSubscriberProcessed($subscriber_id) {
-    $task_subscribers = new TaskSubscribers($this->task()->findOne());
-    return $task_subscribers->isSubscriberProcessed($subscriber_id);
+    if(!empty($this->subscribers)
+      && ScheduledTaskSubscriber::getTotalCount($this->task_id) === 0
+    ) {
+      $subscribers = $this->getSubscribers();
+      return in_array($subscriber_id, $subscribers['processed']);
+    } else {
+      if($task = $this->task()->findOne()) {
+        $task_subscribers = new TaskSubscribers($task);
+        return $task_subscribers->isSubscriberProcessed($subscriber_id);
+      }
+      return false;
+    }
   }
 
   function asArray() {
diff --git a/tests/unit/Models/SendingQueueTest.php b/tests/unit/Models/SendingQueueTest.php
index 292a48a16e..eac1833017 100644
--- a/tests/unit/Models/SendingQueueTest.php
+++ b/tests/unit/Models/SendingQueueTest.php
@@ -3,7 +3,10 @@
 namespace MailPoet\Test\Models;
 
 use AspectMock\Test as Mock;
+use MailPoet\Models\ScheduledTask;
+use MailPoet\Models\ScheduledTaskSubscriber;
 use MailPoet\Models\SendingQueue;
+use MailPoet\Tasks\Sending as SendingTask;
 use MailPoet\Util\Helpers;
 
 class SendingQueueTest extends \MailPoetTest {
@@ -39,6 +42,23 @@ class SendingQueueTest extends \MailPoetTest {
     $mock->verifyInvokedMultipleTimes('decodeEntities', 2);
   }
 
+  function testItChecksProcessedSubscribersForOldQueues() {
+    $subscriber_id = 123;
+    expect($this->queue->isSubscriberProcessed($subscriber_id))->false();
+    $this->queue->subscribers = array('processed' => array($subscriber_id));
+    expect($this->queue->isSubscriberProcessed($subscriber_id))->true();
+  }
+
+  function testItChecksProcessedSubscribersForNewQueues() {
+    $subscriber_id = 123;
+    $queue = SendingTask::create();
+    $queue->setSubscribers(array($subscriber_id));
+    $queue->save();
+    expect($queue->isSubscriberProcessed($subscriber_id))->false();
+    $queue->updateProcessedSubscribers(array($subscriber_id));
+    expect($queue->isSubscriberProcessed($subscriber_id))->true();
+  }
+
   function testItReadsSerializedRenderedNewsletterBody() {
     $queue = $this->queue;
     $data = array(
@@ -99,6 +119,8 @@ class SendingQueueTest extends \MailPoetTest {
 
   function _after() {
     Mock::clean();
+    \ORM::raw_execute('TRUNCATE ' . ScheduledTask::$_table);
+    \ORM::raw_execute('TRUNCATE ' . ScheduledTaskSubscriber::$_table);
     \ORM::raw_execute('TRUNCATE ' . SendingQueue::$_table);
   }
 }
\ No newline at end of file